This invention relates to phenolic foams and particularly to closed-cell foams of a phenol/formaldehyde resin.
The use of foam materials as insulation is an already well-established expedient. However many foam materials that are currently in use have certain inherent problems such as flammability or the production of noxious gases on partial combustion. For this reason there have been a number of attempts to develop a foam with an inbuilt resistance to burning and at the same time high insulation value.
One of the resins explored as having the desired characteristics for producing a flame-resistant foam is a phenolic resin produced by copolymerization of phenol with formaldehyde using a basic catalyst. Such resins are usually called resoles.
The first stage of the production of a phenolic resole is the formation of intermediates with the formula: ##STR1## and the ratio of x/phenolic ring gives the approximate combined phenol/formaldehyde (F/P) ratio of the resin.
These intermediates may then react to give structures with the following crosslinking groups: ##STR2## structure II then, at high temperatures, reacting to split off formaldehyde and give crosslinking groups like structure (I). Further reaction leads to chain extension and crosslinking via reaction at other locations on the aromatic ring.
The process of crosslinking and chain extension is not complete at the end of the foaming process but has progressed to such an extent that the foam has hardened and may be cut into pieces. The degree of cure, in the absence of added crosslinking agents, is a function of temperature and, to some extent, the time of exposure to that temperature. Thus foams that are exposed to only low temperatures have a low degree of cure.
Unfortunately the problems of producing a phenolic foam are substantial in that, if good thermal conductivity is to be maintained, substantially all the cells must be and remain closed-cells. This is not easy since the reaction of phenol with formaldehyde generates water as a by-product and this can easily blow open the cells and so diminish the effectiveness of the foam as a thermal barrier.